As I noted in the comments to Todd's post, in addition to the many benefits of memory-based architectures which Todd lists, there is also the cost benefits in the cloud, which I discussed in Cloud Pricing and Application Architecture.

Why are Memory Based Architectures so attractive? Compared to disk
RAM is a high bandwidth and low latency storage medium. Depending on
who you ask the bandwidth of RAM is 5 GB/s. The bandwidth of disk is
about 100 MB/s. RAM bandwidth is many hundreds of times faster. RAM
wins. Modern hard drives have latencies under 13 milliseconds.
When many applications are queued for disk reads latencies can easily
be in the many second range. Memory latency is in the 5 nanosecond
range. Memory latency is 2,000 times faster. RAM wins again.

RAM is the New Disk

The superiority of RAM is at the heart of the RAM is the New Disk paradigm. As an architecture it combines the holy quadrinity of computing:

Performance is better because data is accessed from memory instead of through a database to a disk.

Scalability
is linear because as more servers are added data is transparently load
balanced across the servers so there is an automated in-memory sharding.

Availability is higher because multiple copies of data are kept in memory and the entire system reroutes on failure.

Application development is faster because there’s only one
layer of software to deal with, the cache, and its API is simple. All
the complexity is hidden from the programmer which means all a
developer has to do is get and put data.

As I noted in the comments to Todd's post, in addition to the many benefits of memory-based architectures which Todd lists, there is also the cost benefits in the cloud, which I discussed in Cloud Pricing and Application Architecture.

Why are Memory Based Architectures so attractive? Compared to disk
RAM is a high bandwidth and low latency storage medium. Depending on
who you ask the bandwidth of RAM is 5 GB/s. The bandwidth of disk is
about 100 MB/s. RAM bandwidth is many hundreds of times faster. RAM
wins. Modern hard drives have latencies under 13 milliseconds.
When many applications are queued for disk reads latencies can easily
be in the many second range. Memory latency is in the 5 nanosecond
range. Memory latency is 2,000 times faster. RAM wins again.

RAM is the New Disk

The superiority of RAM is at the heart of the RAM is the New Disk paradigm. As an architecture it combines the holy quadrinity of computing:

Performance is better because data is accessed from memory instead of through a database to a disk.

Scalability
is linear because as more servers are added data is transparently load
balanced across the servers so there is an automated in-memory sharding.

Availability is higher because multiple copies of data are kept in memory and the entire system reroutes on failure.

Application development is faster because there’s only one
layer of software to deal with, the cache, and its API is simple. All
the complexity is hidden from the programmer which means all a
developer has to do is get and put data.